OSTREA VIRGINIANA. 123 



adductor? Notice the foot and compare it with the foot of 

 Venus, 



How are the gills attached to the body? What would be 

 the effect on the gills if they were attached to the mantle and to 

 each other, as in most forms, when water is ejected in swimming. 



Examine the structure of the gill and notice how much 

 larger the inter-filament ar junctions are near the inter-lamellar 

 junctions than elsewhere. Near the margins of the gills the 

 junctions are frequently simple bunches of cilia, as in Mytilus. 

 Observe the muscular movements of the gills. The gills of this 

 form need to be muscular so they can be drawn together when 

 the animal swims. 



Drawings to show the arrangement of the organs and the struc- 

 ture of the gill are desirable. 



Belding: The Scallop Fishery of Massachusetts. Mass. Fish and Game 



Com., 1910. 

 Drew : The Habits, Anatomy, and Embryology of the Giant Scallop, Pecten 



tenuicostatus. Univ. of Maine Stud., No. 6, 1906. 



OSTREA VIRGINIANA. (Oyste*.) 



This also belongs to the order Pseudo-lamellibranchia. It 

 forms a good example of adaptations for a sedentary life. It 

 occurs, fastened to rocks and other shells, in positions where it is 

 much exposed to attacks of the enemies of lamellibranchs. 



1. Notice the difference in the size and shape of the valves. 

 Why is this desirable ? 



2. Notice the thickness of the valves and the completeness 

 with which they come in contact when the shell is closed. 

 Would such a heavy or tight-closing shell be satisfactory for the 

 scallop or the razor-shell clam? 



3. Open the shell by breaking the edge, inserting a knife- 

 blade through the opening, and cutting the adductor muscle 

 away from the flattened left va\ve of the shell and notice the 

 single adductor, extensive gills, and the absence of a foot. The 

 larval oyster has a foot, but this is lost early in life. 



Brooks: The Oyster. 



Grave: Maryland Shell-Fish Commission, 4, Rep., 1912. 

 Horst: On the Development of The European Oyster (Ostrea edulus, L.). 

 Quart. Jour. Mic. Sci., 22, 1882. 



